1. Field of the Invention
The invention relates to a method for screwing two metal parts together.
2. The Prior Art
In the design of a two-part piston consisting of a crown part and a skirt part, which are screwed together, attention is frequently paid to screwing the crown part and the shaft part to one another by way of regions that are elastically resilient and deform when the two parts are screwed together, so that a bias is exerted on the screw connection, which increases the strength of the screw connection.
German Publication DE 10 2005 060 548 A1 describes a two-part piston that is screwed together. The crown part has a bolt that has an outside thread and is disposed coaxial to the piston axis, on the side facing away from the piston crown. The skirt part is delimited, on the piston crown side, by an elastically resilient ceiling region, which has an opening that lies in the center, into which the bolt is introduced when the crown part and the skirt part are screwed together, before a nut is screwed onto the bolt. The ceiling region deforms, so that a bias is exerted on the screw connection between bolt and nut, which bias improves the strength of the screw connection.
However, a great torque must be exerted on the nut in order to bring about deformation of the ceiling region of the skirt part. Since the nut rests against the ceiling region, this torque must also serve to overcome the friction resistance that occurs during the relative movement between nut and ceiling region. This friction resistance is dependent on the composition of the contact regions by way of which the nut and the ceiling region lie against one another. If the composition of the contact regions is disadvantageous, the torque to be applied for screwing the piston together is incorrectly estimated at an overly great friction resistance between nut and ceiling region, so that the screw connection can come loose during engine operation, and an engine defect can occur.